The pathogenesis of the accumulation of lung water in the variety of states of pulmonary edema is indeed a perplexing problem. Qualitative tests, such as pulmonary function tests, have not been successful as a means of detecting mild increases in lung water. Although several invasive techniques are available, they are not practical to use in a routine clinical setting or for research purposes with volunteers. The existing noninvasive techniques have not to date proved satisfactory. The object of this research is to develop a reproducible, sensitive, accurate and noninvasive technique for measuring lung tissue volume, which is a measure of the lung tissue water. The technique is based on the hypothesis that the uptake of multiple inert gases with a spectrum of solubilities, yields a reliable estimate of lung tissue volume. The proposed technique utilizes the measurement of uptake slopes and hence avoids the errors inherent in the traditional technique of extrapolation to an intercept at time zero. The technique will be implemented on a newly developed computer controlled quadrupole mass spectrometer, and will be assessed for reproducibility using breathholding, rebreathing and open circuit breathing maneuvers. Studies in dogs will assess sensitivity and accuracy. The technique will be used in studies of normals, patients with suspected mild pulmonary edema and patients with maldistribution of ventilation/perfusion. Theoretical studies will assess the reliability of the technique in maldistribution and the possibility of assessing as index of maldistribution.